1. Field of the Invention
This invention relates to adhesive beads and, more particularly, to adhesive beads which comprise a pressure sensitive adhesive core material and a nontacky coating that surrounds the core.
2. Description of the Related Art
Microencapsulated adhesive beads are generally understood to comprise a continuous shell which surrounds or encapsulates a liquid or solid adhesive core. The shell is impervious to the core material and is sufficiently strong so as to prevent exposure of the core during normal handling of the beads. However, upon the application of heat, pressure, mechanical force or the like, the shell fractures, ruptures, dissolves or is absorbed by the core, thereby exposing the core. Microencapsulation is discussed in Microcapsules and Microencapsulation Techniques by M. H. Gutcho (published by Noyes Data Corporation, Park Ridge, N.J., 1976) and Microcapsule Processing and Technology by A. Kondo (edited by J. W. Van Valkenburg, published by Marcel Dekker, Inc., New York, N.Y., 1979).
Two commonly employed techniques for producing microencapsulated adhesive beads are coacervation and in situ polymerization. In coacervation, a continuous shell is formed when a water soluble polymer is condensed from an aqueous solution. The shell forms about a nucleus of material which becomes the core. Shells based on gelatin and gum arabic are well known.
For example, U.S. Pat. No. 2,907,682, "Adhesive Tape Containing Pressure Rupturable Capsules," issued Oct. 6, 1959 to H. J. Eichel discloses an adhesive tape comprising a web having a coating of two kinds of pressure-rupturable capsules thereon. One type of capsule contains a liquid solvent; the other contains a substantially solid adhesive which is soluble in the solvent. When pressure is applied to the tape, the capsules rupture and the adhesive and the solvent become mixed. The capsules include a hard shell formed by coacervation from gelatin and gum arabic.
U.S. Pat. No. 2,988,460, "Adhesive Tape," issued Jun. 13, 1961 to H. J. Eichel discloses an adhesive tape comprising a web coated with pressure-fracturable capsules. Each capsule includes a hard shell which surrounds an adhesive core. The capsules are formed by coacervation. Upon the application of pressure the shells fracture, and with the subsequent application of heat above 100.degree. F. (37.8.degree. C.) the adhesive cores become tacky and flow. U.S. Pat. No. 2,988,461, "Adhesive," issued Jun. 13, 1961 to H. J. Eichel is similar to the immediately preceding patent except that the application of heat is apparently not required to activate the adhesive.
Japanese Kokai Patent No. 63-273680, "Capsule Type Adhesive and Adhesion Method Using Capsule Type Adhesive," published Nov. 10, 1988 discloses an oil with an adhesive dissolved therein. The oil is sealed in a gelatin capsule. Applying pressure to the capsule causes the same to break thereby releasing the oil/adhesive blend.
Japanese Patent Publication No. 60-124679, "Pressure Sensitive Adhesive Sheet," published Jul. 3, 1985 discloses several adhesive microcapsules. For example, drawing FIG. 2(d) illustrates a pressure sensitive adhesive core covered by a fine inorganic powder and then encapsulated by a polymer film which is obtained by coacervation. Pressure is applied to the microcapsule to expose the adhesive core.
In situ polymerization is a second commonly employed technique for producing microencapsulated adhesive beads. A shell formed of a gaseous, liquid, water or oil soluble monomer or a low molecular weight polymer is polymerized on the surface of a core material to provide a polymer film which covers the entire surface of the core material. A variety of materials including homopolymers, copolymers, graft copolymers and block copolymers may be used to form the shell. Shells based on urea-formaldehyde are well known.
For example, British Patent Specification No. 989,264, "Microcapsules and Method of Producing Them," published Apr. 14, 1965 discloses microcapsules comprising discrete, distinct and continuous aminoplast shell walls upon water-immiscible inert solid or liquid fill particles. In situ polymerization is also mentioned in Japanese Kokai Patent No. 2-102280, "Microencapsulated Pressure Sensitive Adhesive Agent," published Apr. 13, 1990 which discloses a pressure sensitive adhesive agent and a non-pressure sensitive adhesive shell which surrounds the agent. Japanese Kokai Patent No. 2-292380 contains similar disclosure.
Adhesive beads are discussed in other publications. For example, U.S. Pat. No. 4,091,162, "Adhesives," issued May 23, 1978 to Henderson et al. discloses a "core-shell" polymer particle comprising a soft, tacky polymeric core surrounded by a hard, nontacky, nonblocking polymeric shell. The particles may be formed by polymerizing the core followed by polymerizing the shell about the core. A typical "core-shell" polymer particle is illustrated in FIG. 1 of the Henderson et al. patent. Japanese Kokai Patent No. 2-102280 mentions a similar technique for producing a structure which includes a pressure sensitive adhesive agent and a nonadhesive shell.
The above-mentioned Japanese Patent Publication No. 60-124679 discloses three adhesive microcapsules other than the coacervate structure illustrated in drawing FIG. 2(d). Drawing FIG. 2(a) shows an adhesive microcapsule in which a frozen and ground pressure sensitive adhesive is mixed with a resinlike or terpenelike resin to form a powder that reportedly flows well at room temperature. The microcapsule illustrated in drawing FIG. 2(b) apparently comprises the microcapsule of FIG. 2(a) further covered with a fine inorganic powder such as silica, bentonite, alumina or talc to enhance the flowability of the microcapsules. The microcapsule of drawing FIG. 2(c) comprises a pressure sensitive adhesive core covered with a fine inorganic powder.
U.S. Pat. No. 4,833,179 and its division U.S. Pat. No. 4,952,650, "Suspension Polymerization," each to Young et al. and issued, respectively, May 23, 1989 and Aug. 28, 1990 disclose the production of pressure sensitive adhesive beads by suspension polymerization. The beads may include a hydrophobic silica coating. A filtration product comprising the beads and water is nonagglomerating but the beads are inherently and permanently tacky when dried.
It would be advantageous to provide beads having pressure sensitive adhesive qualities but which do not have a tacky exterior surface. Such beads could be more easily processed by spray drying equipment or supplied to a hopper feeder. Furthermore, these beads would be free from packaging disabilities associated with 100% solids pressure sensitive adhesives in that special release liners or pail or drum unloaders would not be required to deliver the beads from their packing container for use. Moreover, if the beads were a 100% solids system, they could be readily applied to a substrate by dusting or other dry coating techniques, thereby eliminating the need for aqueous or solvent based suspensions that must be dried once the adhesive has been applied. Such beads, however, should be easily activated and not rendered nontacky in a manner that detrimentally affects their ultimate adhesion.